The boldly striped red and white lionfish is popular in saltwater aquariums. But don’t let its pretty fins fool you. These predatory fish boast a huge appetite and spines full of venom. And they are a menace outside their native waters (coral reefs in the Indian Ocean and waters near Australia and Japan). They have been invading waters off of the U.S. East coast. Work launched by a middle-school student now shows that lionfish aren’t limited to seawater. This could be bad news for fish living in coastal rivers and other regions where fresh and salty water mix.

Lauren Arrington is a rising eighth grader at The Kings Academy in West Palm Beach, Fla. For her sixth grade science fair project, she tested how salty the water had to be for lionfish to survive. Her findings: These invaders can survive in water that is almost fresh.

Her results caught the attention of local scientists. Now, they have published a paper, based on her lionfish project. It appeared earlier this year in the scientific journal Environmental Biology of Fishes.

A chance observation

It all started when Lauren, now 13, was enjoying the rivers near her home in Jupiter, Fla. “I was seeing lionfish more and more when we fished and snorkeled,” she recalls.The teen began to wonder why this saltwater animal was showing up in places containing lots of freshwater.

The lionfish (Pterois volitans and Pterois miles) that have been invading U.S. and Caribbean waters are ocean species. That means they should need saltwater to survive. If water doesn’t have enough salt, marine fish can begin to lose salt through their skin. It’s the result of a process called osmosis.

Fish skin is permeable, so salts can pass through it fairly easily. A fish can get the salt it needs from the water around it — unless the water is less salty than its tissues. When that happens, a saltwater fish can begin losing its salt to the water around it. And without salts to help run its cells, this fish can die.

Lauren reasoned this is why lionfish shouldn’t be showing up in rivers and estuaries. The water shouldn’t be salty enough. Yet they were there anyway. Somehow they could tolerate very low levels of salt. Lauren wanted to find out just how low a salt level — water salinity — these lionfish could stand.

For her experiments, Lauren caught six lionfish. She placed each in its own 37.8-liter (10 gallon) tank. (The local community river center, which uses aquariums to teach visitors about the animals and plants that live in the Loxahatchee River, let Lauren use its tanks and space at no cost.) She filled each tank with water containing salt at a concentration of 25 parts per thousand (ppt) parts of water. Over the next 16 days, she poured fresh water into five of the tanks every night. This gradually made the water less salty. The sixth tank remained at the starting salinity level. That made it a control.

A control is a portion of an experiment where nothing changes. Comparing new conditions to a control is vital to science. It will show that only the part of a test that a scientist is changing can be responsible for the effects.

Every day, Lauren fed her animals and checked her tanks. To her surprise, even though the water became much less salty, her lionfish appeared to be just fine. They were even able to survive overnight in water that had only six ppt salt. That level is very close to fresh water. Fresh water has less than one ppt salt.

At that point, Lauren stopped her testing. Lauren didn’t want to risk killing off any of her fish by accident. Her school’s science fair had rules stating that no animals could die as a result of any project.

A scientist noticed her work

Before long, Zack Jud heard about the teen’s results. He’s a marine ecologist at Florida International University in North Miami. He also had seen lionfish in Florida’s estuaries and rivers, and published some of his findings in 2011.Some of the fish showed up more than 6.4 kilometers (4 miles) upstream from the ocean. Lauren’s findings gave him a starting point for more experiments.

Jud collected 24 lionfish and housed them in underwater cages in estuaries and rivers. The different test sites had different salinities. This allowed him to see how well lionfish survived at different salt levels in outside the lab. He took 16 more lionfish back to his lab where he could repeat his experiments.

Jud showed that Lauren was right. Lionfish can survive in low salt conditions for long periods. In his study, they lived for 28 days at salt levels of seven ppt. That’s one-fifth the saltiness of seawater. The fish could even survive very short periods in 1 ppt salt conditions. “That’s almost pure fresh water,” he notes — just one thirty-fifth the saltiness of seawater.

While this ability to tough it out in low-salt water might be good for lionfish, it could pose a big threat to the native fish who call these waters home. Lionfish are not picky eaters. The native fish they encounter can make a great buffet.

Craig Layman is a marine biologist at North Carolina State University in Raleigh. He helped out with Jud’s study. The biggest threat from lionfish is that they might eat young native fish, he says. Many species of fish along the coastal United States breed in estuaries. If lionfish spread into estuaries and rivers, they could feast on baby fish. “We suspected that lionfish were living in low salinities,” he says. “Lauren’s experiment showed us for sure.”

Of course, lionfish aren’t the only tough ocean fish. Christie Wilcox is a marine biologist at the University of Hawaii in Manoa. “Just about all marine fish can survive very low saltwater concentrations for a while,” she notes.

While lionfish might swim up into estuaries, they probably won’t live there full time, Jud suspects. “Lionfish eggs float at the surface for a few days before hatching,” he explains. “In many estuaries, there is a surface layer of pure fresh water floating on top of saltier bottom water.” That could be too stressful an environment for a delicate lionfish egg.

While many ocean fish can tolerate water low in salt, they don’t choose to spend time in such areas. That’s why most won’t swim into rivers. But Jud and Lauren both found lionfish in estuary systems. These are places where rivers meet the ocean, and salt and freshwater mix. They should be too fresh to tempt most ocean fish. But the lionfish appeared to be right at home. Thanks to Lauren’s research, scientists now know these lionfish visits aren’t a fluke. And that suggests they’ll need to learn more if they hope to save local species from these voracious predators.

Power Words

brackish A term for water that contains a mixture of saltwater and freshwater.

concentration A measurement of how much of one substance has been dissolved into another.

control A part of an experiment where nothing changes. The control is essential to scientific experiments. It shows that any new effect must be due to only the part of the test that a researcher has altered. For example, if scientists were testing different types of fertilizer in a garden, they would want one section of to remain unfertilized, as the control. Its area would show how plants in this garden grow under normal conditions. And that give scientists something against which they can compare their experimental data.

ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

estuary The mouth of a large river, where it empties into the ocean and freshwater and saltwater mix. Such regions are often nurseries for young fish.

freshwater Water that is not salty, such as rain, rivers and lake water.

invasive species (also known as aliens) A species that is found living, and often thriving, in an ecosystem other than the one in which it evolved. Some invasive species were deliberately introduced to an environment, such as a prized flower, tree or shrub. Some entered an environment unintentionally, such as a fungus whose spores traveled between continents on the winds. Still others may have escaped from a controlled environment, such as an aquarium or laboratory, and begun growing in the wild. What all of these so-called invasives have in common is that their populations are becoming established in a new environment, often in the absence of natural factors that would control their spread. Invasive species can be plants, animals or disease-causing pathogens. Many have the potential to cause harm to wildlife, people or to a region’s economy.

native Associated with a particular location; native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people).

osmosis The movement of certain molecules within a solution across a membrane. The movement is always from where the concentration of some chemical is higher to where the concentration is lower. This movement tends to continue until concentrations on each side of the membrane are the same.

permeable Having pores or openings that permit liquids or gases to pass through. Sometimes materials can be permeable for one particular type of liquid or gas (water, for example) but block others (such as oil).

saline Salty or having to do with salt. The salinity of the ocean is a measure of its saltiness.

Editor's note: Updated July 22, 2014 to clarify that Jud's work began prior to Lauren's lionfish research.